This invention relates to liquid level sensors, and more particularly to an improved device for sensing the level of a fluid in an automotive engine or the like. Even more particularly, this invention relates to a novel dipstick which incorporates electrical means for sensing the level of the oil in the engine block or crackcase of an automotive vehicle.
From their early inception, most automobile engines have included a conventional metal dipstick for manually checking the level of the lubricating oil in the engine block. In recent years efforts have been made to provide improved sticks, such as for example those having electrical sensing means for indicating the level of the oil in the block. For example, U.S. Pat. No. 4,186,604 suggests the use of a helically coiled strand, which is adapted to be inserted in the engine block dipstick tube in place of the conventional dipstick. A thermistor, which is secured to the lower end of the coiled strand, is connected to an electric circuit that provides a visual indication of when the oil in the block has dropped below a predetermined level. This invention was alleged to have been an improvement over the known practice of using heat shrinkable tubing to attach a thermistor to a lower end of a conventional dipstick.
U.S. Pat. No. 3,098,914 also suggests the use of a single, temperature-responsive thermistor switch on a transmission dipstick for detecting the level of oil in an automatic transmission. When the level of fluid in the transmission level drops below a predetermined height, a switch closes to actuate a warning light on the dashboard of the vehicle. If the level is high enough to cover the switch, the switch opens once the oil has been warmed up by driving the auto. In this mechanism the switch is merely secured to the side of a conventional dipstick, as are the wires which are employed for connecting the switch to the temperature-responsive circuit.
U.S. Pat. No. 2,780,692 discloses a specially shaped transmission dipstick having a perforated tubular member attached to the lower end thereof. A float, which shifts up and down in the tubular member in response to changes in the level of the transmission fluid, controls the operation of a switch that controls a warning device.
Although not specifically designed for an automotive engine block or transmission, the liquid level detector disclosed in U.S. Pat. No. 3,500,687 employs two spaced thermistors, which are mounted in a housing that extends downwardly into a reservoir to monitor the amount of liquid in the reservoir. When the liquid in the reservoir falls below a predetermined level, the upper thermistor begins to heat up, thereby to operate a first warning lamp. If the liquid falls below the second thermistor a second warning lamp is illuminated.
While the above-noted patents disclose various ways of utilizing thermistors for sensing the level of fluids, most such devices are either impractical or do not provide the accuracy which is desirable for proper monitoring of the oil level in automobiles and the like. For example, if a single thermistor is employed it is likely to give an erratic indication of the true level of the oil in the enginer block, as for example when the oil sloshes or shifts and momentarily exposes the thermistor, thus causing energization of the associated warning device even though the true level of the oil might be satisfactory. Although the above-noted U.S. Pat. No. 3,500,687 discloses a circuit which would miniminize this problem, nevertheless this particular patent does not disclose a satisfactory device for retrofitting a conventional automotive dipstick with an electrically operated sensor.
Still another disadvantage of prior art devices such as those referred to above is that, in most cases where electrically operated sensing devices have been incorporated in dipsticks, such devices are effective only during certain operating conditions of an automobile vehicle, and are not reliable at other times.
It is an object of this invention, therefore, to provide an improved engine oil level sensor of the type which includes electrical sensing means, and which enables either automatic or manual checking of the oil level in the engine.
Another object of this invention is to provide an improved oil level dipstick and electrical sensor therefor which are relatively inexpensive to manufacture, and which can be readily substituted in place of conventional automotive engine dipsticks.
Still another object of this invention is to provide an improved dipstick of the type described, which is adapted to be removably inserted in the dipstick tube of an engine block for ready removal and manual inspection, and which has incorporated therein electrical sensing means for providing automatic monitoring of the level of the oil in the block.